Edit his answer so I'll be able to undo my downvote? Sounds like a workaround, not a solution.
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sq33GMar 11 '14 at 12:57

1

@sq33G only solution is totally removing the block and allowing one to change votes without any restriction, and that would cause much more harm than good thus will never happen. So all we have are workarounds.
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Shadow WizardApr 15 at 14:14

If there is nothing to improve in the question (thus edit won't be proper), you can apologize to the OP by a comment and ask that if and when edit occurs, notify you as well (via comment reply) so that you can reverse the vote.

Also, you can go through other posts of that user (questions or answers) and upvote what you deem correct and helpful.

All in all, it's really not the end of the world - mistakes happen, single downvote won't really change anything.